Gate drive circuit, power conversion apparatus, and railway vehicle

ABSTRACT

A gate drive circuit to prevent a false turn-on phenomenon includes a first, second, third and fourth switching element, and a capacitor. A source of the first switching element is connected to a first voltage, and a drain of the same is connected to the main switching element&#39;s gate electrode. A source of the second switching element is connected to a second voltage, and a drain of the same is connected to the gate electrode. A source of the third switching element is connected to the first voltage, and a drain of the same is connected to a first electrode of the capacitor. A source of the fourth switching element is connected to the second voltage, and a drain of the same is connected to the first electrode and to the drain of the third switching element. A second electrode of the capacitor is connected to the gate electrode.

TECHNICAL FIELD

The present invention relates to a gate drive circuit, a powerconversion apparatus, and a railway vehicle. More particularly, thepresent invention relates to a technique effectively applied to adriving circuit that drives a power device using a silicon carbidematerial, etc.

BACKGROUND ART

For example, Patent Document 1 describes a configuration in which, inorder to simply switch a switching speed (dv/dt: voltage change rate) ofan insulated-gate bipolar transistor (IGBT) between a high-speed modeand a low-speed mode, a capacitor and a switch are connected in seriesbetween a gate and an emitter of this insulated-gate bipolar transistor.

Patent Document 2 describes a configuration in which, in order to detectan abnormal current flowing through a switching element for powerconversion to protect this switching element, a capacitor and aprotective switching element are connected in series between a gate andan emitter of the switching element.

RELATED ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Laid-open

Patent Document 2: Japanese Patent Application Laid-open Publication No.2009-213305

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

For example, as application of a power device, a so-called inverterdevice (DC/AC converter device) shown as an inverter 13 in FIG. 1 of theabove-described Patent Document 2 is generally cited. In an inverterdevice, a reflux diode element and a switching element made of a powerdevice are connected in series between a power supply on to high-voltageside (upper arm) and a power supply on a low-voltage side (lower arm).These switching elements of the upper and lower arms are alternatelyturned on and off, so that a DC level at the former stage of theinverter is converted into an AC level, and the AC is supplied to a loadcircuit such as an AC isolation transformer and a motor at the latterstage.

As an element expected to be used as such a switching element, an SiCMOSFET and an SiC IGBT using silicon carbide (SiC) are cited. The SiCMOSFET and the SiC IGBT are almost the same in an element structure withan existing power device such as an Si MOSFET and an Si IGBT usingsilicon (Si), and are the same in a method of driving them therewith. Inother words, for the SiC element, an existing gate drive circuit for theSi element can be basically used, and therefore, the SiC element isconvenient.

Further, the SiC element has a lower on-resistance than that of the Sielement, and therefore, also has an advantage capable of reducing a lossresulting from an inverter operation. However, the SiC element has asmaller substrate film thickness than that of the Si element, andtherefore, has such a problem as so-called large input capacitance.

Therefore, by the usage of the driving circuit for the Si element as thedriving circuit for the SiC element without careful consideration, theinput capacitance is increased, and, as a result, the switching speed isdecreased in some cases. The decrease in the switching speed leads toincrease in the switching loss, and there is a possibility ofdeterioration in a conversion efficiency of a power conversionapparatus.

As means for preventing the decrease in the switching speed, decrease ina value of a gate resistance connected in series to a gate terminal ofthe switching element is generally cited. By the decrease in the valueof the gate resistance, the input capacitance of the switching elementcan be charged/discharged at a high speed, and therefore,controllability of the switching speed of an element with a large inputcapacitance such as the SiC element is improved.

However, when the switching speed becomes high by the decrease in thegate resistance value, the input capacitance is charged/discharged in ashort time, and therefore, a gate current significantly increases. Theincrease in the gate current causes a risk of deterioration of aresistor and a capacitor which are passive elements in the gate drivecircuit. Further, it is required to enhance an output current at anoutput stage of the gate drive circuit in accordance with the increasein the gate current, and therefore, there is a problem of increase in acost of the gate drive circuit.

As means for solving such problems, a method of switching the switchingspeed mode as disclosed in the above-described Patent Document 1 iscited. According to the method of the Patent Document 1, an externalcapacitor does not effectively function as a capacitance during ahigh-speed switching mod, and therefore, charge/discharge currents ofthe external capacitor is reduced, and, as a result, a total gatecurrent can be reduced.

Meanwhile, according to the method disclosed in the Patent Document 1,the capacitive value of the external capacitor is effectively reduced inthe high-speed switching mode, and therefore, such a problem asso-called false turn-on phenomenon cannot be solved.

The false turn-on phenomenon is caused when, for example, a state of theupper arm of the inverter changes from the off-state to the on-state asthe lower arm is turned off off. In this case, a drain voltage at thelower arm rapidly increases, so that the charge/discharge current flowin the capacitance between the gate and the drain of the switchingelement of the lower arm. As a result, a voltage between the gate andthe source of the switching element of the lower arm increases from itsvoltage level of the off-state. Once the voltage level exceeds athreshold of the switching element, the switching element of the lowerarm is erroneously turned on, a state of which is supposed to be theoff-state.

As described above, the false turn-on phenomenon is a phenomenon thaterroneously turns on the switching element, a state of which is supposedto be the off-state. The false turn-on phenomenon may be caused alsowhen the Si element is used as the switching element of the lower arm.However, particularly when the SiC element is used, the inputcapacitance, particularly a feedback capacitance, is large, andtherefore, the above-described charge/discharge current flowing in thecapacitance between the gate and the drain becomes large, and the falseturn-on phenomenon tends to occur.

An object of the present invention has been made in consideration of theabove-described circumstances, and is to provide a technique capable ofsuppressing a gate driving current to prevent a false turn-on phenomenonin a driving circuit that drives a power device.

The above and other objects and novel characteristics of the presentinvention will be apparent from the description of the presentspecification and the accompanying drawings.

Means for Solving the Problems

The summary of the typical aspects of the inventions disclosed in thepresent application will be briefly described as follows.

A gate drive circuit according to one embodiment is a gate drive circuitdrives a main switching element. The gate drive circuit includes a firstswitching element, a second switching element, a third switchingelement, a fourth switching element, and a capacitor. A source of thefirst switching element is connected to a first voltage, and a drain ofthe same is connected to a gate electrode of the main switching element.A source of the second switching element is connected to a secondvoltage, and a drain of the same is connected to the gate electrode ofthe main switching element. A source of the third switching element isconnected to the first voltage, and a drain of the same is connected toa first electrode of the capacitor. A source of the fourth switchingelement is connected to the second voltage, and a drain of the same isconnected to the first electrode of the capacitor and to the drain ofthe third switching element. A second electrode of the capacitor isconnected to the gate electrode of the main switching element.

A power conversion apparatus according to the one embodiment includes:three paired first main switching elements and second main switchingelements, each pair of which are for a U phase, a V phase, or a W phaseand are connected in series between a power supply on a high-voltageside and a power supply on a low-voltage side; and three paired firstgate drive circuits and second gate drive circuits, each pair of whichare for the U phase, the V phase, or the W phase and alternately turneach of the three paired first and second main switching elements on andoff. Each of the first gate drive circuits and the second gate drivecircuits is the above-described gate drive circuit according to the oneembodiment.

A railway vehicle according to the one embodiment is a railway vehiclehaving a power converter driving a three-phase motor. The powerconverter includes an inverter converting a direct-current power, whichis created by converting an alternate-current power that is input froman alternate-current overhead wire, into an alternate-current power tobe supplied to the three-phase motor. The invertor includes: threepaired first main switching elements and second main switching elements,each pair of which are for a U phase, a V phase, or a W phase and areconnected in series between a power supply on a high-voltage side and apower supply on a low-voltage side; and three paired first gate drivecircuits and second gate drive circuits, each pair of which are for a Uphase, a V phase, or a W phase and alternately turn the three pairedfirst and second main switching elements on and off. Each of the firstgate drive circuits and the second gate drive circuits is theabove-described gate drive circuit according to the one embodiment.

Effects of the Invention

The effects obtained by the typical aspects of the present inventionwill be briefly described below.

(1) In a drive circuit that drives a power device, a gate drive currentcan be suppressed.

(2) A false turn-on phenomenon can be prevented.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is explanatory diagrams each showing an example of asemiconductor switching element used in a gate drive circuit accordingto an embodiment, in which (a) shows an SiCIGBT and (b) shows an SiCMOS;

FIG. 2 is explanatory diagrams each showing an example of across-sectional structure of the semiconductor switching element of FIG.1, in which (a) shows a cross-sectional structure of the SiCIGBT and (b)shows a cross-sectional structure of the SiCMOS;

FIG. 3(a) is an explanatory diagram showing an example of a capacitiveproperty of the semiconductor switching element of FIG. 1, and FIG. 3(b)is an explanatory diagram showing a capacitive property of aconventional semiconductor switching element using a silicon material;

FIG. 4 is a circuit diagram showing an example of an operation mode (t1)of the gate drive circuit of the embodiment;

FIG. 5 is a circuit diagram showing an example of an operation mode (t2)of the gate drive circuit of the embodiment;

FIG. 6 is a circuit diagram showing an example of an operation mode (t3)of the gate drive circuit of the embodiment;

FIG. 7 is a circuit diagram showing an example of an operation mode (t4)of the gate drive circuit of the embodiment;

FIG. 8 is a waveform chart showing an example of a timing chart of thegate drive circuit of the embodiment;

FIG. 9 is an explanatory diagram showing an example of a configurationof a power conversion apparatus according to the embodiment; and

FIG. 10 is an explanatory diagram showing an example of a configurationof an alternate-current type railway vehicle according to theembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

In the embodiments described below, the invention will be described in aplurality of sections or embodiments when required as a matter ofconvenience. However, these sections or embodiments are not irrelevantto each other unless otherwise stated, and the one relates to the entireor a part of the other as a modification example, details, or asupplementary explanation thereof.

Also, in the embodiments described below, when referring to the numberof elements (including number of pieces, values, amount, range, and thelike), the number of the elements is not limited to a specific numberunless otherwise stated or except the case where the number isapparently limited to a specific number in principle. The number largeror smaller than the specified number is also applicable.

Further, in the embodiments described below, it goes without saying thatthe components (including element steps) are not always indispensableunless otherwise stated or except the case where the components areapparently indispensable in principle.

Similarly, in the embodiments described below, when the shape of thecomponents, positional relation thereof, and the like are mentioned, thesubstantially approximate and similar shapes and the like are includedtherein unless otherwise stated or except the case where it isconceivable that they are apparently excluded in principle. The samegoes for the numerical value and the range described above.

Also, the same components are denoted by the same reference symbols inprinciple throughout all the drawings for describing the embodiments,and the repetitive description thereof is omitted. In order to make thedrawings easy to see, note that hatching is used even in a plan viewwhile hatching is omitted even in a cross-sectional view in some cases.

In the embodiments, note that a MOSFET (Metal Oxide Semiconductor FieldEffect Transistor) is used as an example of a MISFET (Metal InsulatorSemiconductor Field Effect Transistor). However, a non-oxide filmserving as a gate insulating film is not excluded.

The embodiments will be described in detail based on drawings below. Inorder to easily understand features of the embodiments, a margin forimprovement existing in related arts will be described first.

[Margin for Improvement]

In a major social trend of global environment preservation, importanceof electronics business that reduces environmental loads has increased.Power devices are particularly used for inverters of railway vehiclesand hybrid/electric cars, inverters of air conditioners, and powersupplies of consumer products as personal computers, and therefore,improvement in performances of the power devices significantlycontributes to improvement in power efficiencies of infrastructuresystems and consumer products.

The improvement in the power efficiencies means reduction in energyresources required for system operations. In other words, emissionamounts of carbon dioxides can be reduced, that is, environmental loadscan be reduced. For this reason, research and development activitiesaiming at the improvement in the performance of the power device havebeen actively performed.

Generally, the power device is made of silicon (Si) as similar to alarge-scale integrated circuit (LSI). In a power conversion apparatususing such an Si power device, such as an inverter, in order to reduceits energy loss, research and development for achieving properties suchas low on-resistance (Ron), a high current density, and a high breakdownvoltage by optimizing profiles of element structures of diode elementsand switching elements and impurity concentrations thereof have beenactively performed.

In recent years, attention has been paid to compound semiconductors madeof silicon carbide (SiC), gallium nitride (GaN), etc., having a largerband gap than that of silicon, as power device materials. Each of thesecompound semiconductors has such a large bandgap, and therefore, has abreakdown voltage that is about 10 times as high as that of silicon.

For this reason, such a compound device can have a smaller thicknessthan that of a Si device, and have a much lower resistance value (Ron)in a conduction-state. As a result, so-called conduction loss (Ron×i²)expressed by a product of the resistance value (Ron) and a conductioncurrent (i) can be reduced, so that this manner can significantlycontribute to the improvement in the power efficiency. As attention ispaid to such advantages, the diode elements and the switching elementsusing the compound materials have been actively researched anddeveloped.

As application of such a power device, for example, a so-called inverter(DC/AC converter) as shown in an inverter 13 of FIG. 1 of theabove-described Patent Document 2 is generally cited. In an inverterdevice, a switching element made of a power device and a reflux diodeelement are connected in series between a power supply on tohigh-voltage side (upper arm) and a power supply on a low-voltage side(lower arm). These switching elements of the upper and lower arms arealternately turned on and off, so that a DC level at the former stage ofthe inverter is converted into an AC level, and the AC is supplied to aload circuit such as an AC isolation transformer and a motor at thelatter stage.

In this case, as losses caused in the inverter, a conduction loss and arecovery loss based on the on-resistances (Ron) of the switching elementand the diode element as described above, and a switching loss aremainly cited, the switching loss being caused by the switchingoperation, that is, by the current flow between the source and the drainduring a period in which the switching element shifts from its on-stateto off-state or from its off-state to on-state (period in which apotential difference is caused between the drain and the source). Aselements expected to be used as such a switching element, an SiC MOSFET(which will hereinafter be described as SiCMOS) and SiC IGBT (which willhereinafter be described as SiCIGBT) using SIC are cited.

The SiCMOS and SiCIGBT are almost the same in an element structure withexisting power devices such a Si MOSFET and a Si IGBT using silicon, andare the same also in a drive method with them. In other words, anexisting gate drive circuit for the Si element can be basically alsoused for the SiC element, and therefore, is convenient.

Further, the SiC element has a lower on-resistance than that of the Sielement, and therefore, also has an advantage of reduction in the lossesresulting from the inverter operations. However, the SiC element has asmaller substrate film thickness than that of the Si element, thus has aproblem of increase in so-called input capacitance (Ciss).

Therefore, by the usage of the drive circuit for the Si element as thedrive circuit for the SiC element without careful consideration, theinput capacitance is increased, and, as a result, the switching speed(dv/dt) is decreased in some cases. The decrease in the switching speedleads to increase in the switching losses, and therefore, there is apossibility of deterioration of the conversion efficiency of the powerconversion apparatus.

As means for preventing the decrease in the switching speed, decrease ina value of a gate resistor connected in series to the gate terminal ofthe switching element is generally cited. By the decrease in the valueof the gate resistor, the input capacitance of the switching element canbe charged/discharged at a high speed, and therefore, controllability ofthe switching speed of the element such as the SiC element having thelarge input capacitance is improved.

However, by the increase in the switching speed because of the decreasein the value of the gate resistor, the input capacitance ischarged/discharged in a short time, and therefore, a gate currentsignificantly increases. The increase in the gate current causes a riskof deterioration of a resistor and a capacitor which are passiveelements in the gate drive circuit. Further, it is required to enhancean output current at an output stage of the gate drive circuit inaccordance with the increase in the gate current, and therefore, aproblem of increase in a cost of the gate drive circuit arises.

As means for solving such problems, a method of switching the switchingspeed mode as disclosed in the above-described Patent Document 1 iscited. According to the method of the Patent Document 1, an externalcapacitor does not effectively function as a capacitance during ahigh-speed switching mod, and therefore, charge/discharge currents ofthe external capacitor is reduced, and, as a result, a total gatecurrent can be reduced.

Meanwhile, according to the method disclosed in the Patent Document 1,the capacitive value of the external capacitor is effectively reduced inthe high-speed switching mode, and therefore, such a problem asso-called false turn-on phenomenon cannot be solved.

The false turn-on phenomenon is caused when, for example, a state of theupper arm of the inverter changes from the off-state to the on-state asthe lower arm is turned off. In this case, a drain voltage at the lowerarm steeply increases, so that the charge/discharge current flow in thecapacitance between the gate and the drain of the switching element ofthe lower arm. As a result, a voltage between the gate and the source ofthe switching element of the lower arm increases from its voltage levelof the off-state. Once the voltage level exceeds a threshold of theswitching element, the switching element of the lower arm is erroneouslyturned on, a state of which is supposed to be the off-state.

As described above, the false turn-on phenomenon is a phenomenon thaterroneously turns on the switching element, a state of which is supposedto be the off-state. The false turn-on phenomenon may be caused alsowhen the Si element is used as the switching element of the lower arm.However, particularly when the SiC element is used, the inputcapacitance (Ciss), particularly a feedback capacitance (Crss), islarge, and therefore, the above-described charge/discharge currentflowing in the capacitance between the gate and the drain becomes large,and the false turn-on phenomenon tends to occur.

Accordingly, the embodiment provides a contrived technique for theimprovement margin existing in the above-described related arts. Atechnical concept of the contrived embodiment will be explained belowwith reference to drawings. The technical concept of the embodiment isto provide a technique, in a drive circuit that drives a power device,capable of suppressing a gate drive current to prevent a false turn-onphenomenon.

More specifically, the SiC element has a large input capacitance and alarge feedback capacitance, and therefore, has a possibility of thedecrease in the switching speed. Therefore, both of achievement of thehigh-speed switching as suppressing the increase in the gate drivecurrent in order to prevent the increase in the cost of the gate drivecircuit and achievement of such a stable switching operation as notcausing the false turn-on phenomenon are the means for achieving thedecrease in the cost and the decrease in the loss of the entire powerconversion apparatus. Further, a technique for suppressing the gatedrive current, preventing the false turn-on phenomenon, decreasing thelosses, and improving the reliability is provided.

Embodiment

A gate drive circuit, a power conversion apparatus, and a railwayvehicle according to the embodiment will be described with reference toFIGS. 1 to 10. In the embodiment, a semiconductor switching element isreferred to also simply as switching element or switch, and asemiconductor diode element is referred to also simply as diode elementor diode.

<Configuration of Gate Drive Circuit>

FIG. 1 is explanatory diagrams each showing an example of thesemiconductor switching element used in the gate drive circuit accordingto the embodiment. In FIG. 1, FIG. 1 (a) shows the SiCIGBT and FIG. 1(b) shows an SiCMOS as an example of the switching element SW asexamples of a switching element SW.

The SiCIGBT is a switching element SW with three terminals having a gateelectrode G, a collector electrode C, and an emitter electrode E. TheSiCMOS is a switching element SW with three terminals having a gateelectrode G, a drain electrode D, and a source electrode S. Inapplication to a gate drive circuit of a power conversion apparatusdescribed later (in FIG. 9), the switching element SW shown in FIG. 1(a) is shown with a diode element D (SiC PND) necessary for a refluxoperation in the drawing. The switching element SW (SiCMOS) shown inFIG. 1 (b) has a built-in diode element therein in its structure, andtherefore, is shown with a diode element D (SiC build-in PND).

FIG. 2 is explanatory diagrams each showing an example of across-sectional structure of the semiconductor switching element ofFIG. 1. In FIG. 2, FIG. 2(a) shows a cross-sectional structure of theSiCIGBT and FIG. 2(b) shows a cross-sectional structure of the SiCMOS soas to correspond to the switching element SW shown in FIG. 1.

First, the SiCMOS shown in FIG. 2(b) will be described. The SiCMOS is asemiconductor switching element SW formed of a MOSFET made of an SiCcompound material. FIG. 2(b) shows a SiCMOS of so-called DMOS (DoubleDiffusion Metal Oxide Semiconductor) type. In FIG. 2(b), SPm denotes asource electrode, GPm denotes a gate electrode, DRm denotes a drainelectrode, SUB denotes a substrate, Tox denotes a gate insulating film,N⁺ denotes a source layer, P denotes a base layer, and DFT denotes adrift layer.

In the SiCMOS shown in FIG. 2(b), the source layer N⁺, which is formedas an n⁺-type region connected to the source electrode SPm, is connectedto the drift layer DFT via a channel formed in the base layer P which isformed as a p-type region. The drift layer DFT is, for example, ann⁻-type region, and plays a role of securing a breakdown voltage. Thesubstrate SUB is, for example, an n⁺-type region, and the drainelectrode DRm is connected to the substrate SUB. Although not shown inthe drawings, note that the source electrode SPm, the gate electrodeGPm, and the drain electrode DRm are connected to electrode pads,respectively, by using metal wiring layers. The base layer P and thedrift layer DFT have a function of the built-in diode element. ThisSiCMOS has a simple element structure, and therefore, has an advantageof a lower manufacturing cost than that of an SiCMOS of a trenchstructure type. Therefore, a power converter with a low cost and a lowloss can be achieved.

The SiCIGBT shown in FIG. 2(a) is a semiconductor switching element SWformed of an IGBT (Insulated Gate Bipolar Transistor) made of an SiCcompound material. The SiCIGBT of FIG. 2(a) is different from the SiCMOSin that the source electrode SPm and the drain electrode DRm arereplaced with the emitter electrode EPm and the collector electrode CRm,respectively, and in that a buffer layer N⁺Buf and a high-concentrationP⁺ layer with a are formed between the drift layer DFT and the substrateSUB. Because of the existence of the high-concentration P⁺ layer, aconductivity modulation phenomenon is caused, and the on-resistancedrastically decreases. By application of the buffer layer N⁺Buf asneeded, the switching loss of the element can be decreased. In thismanner, the SiCIGBT structure is more complicate and requires a highercost for element formation than those of the SiCMOS structure. However,when the SiCIGBT structure is used for a converter for a railwayvehicle, etc., requiring a large current, the SiCIGBT structure has anadvantage of reducing the loss of the converter because theon-resistance and the switching can be decreased.

FIG. 3 (a) is an explanatory diagram showing an example of thecapacitive property of the semiconductor switching element of FIG. 1.And, FIG. 3(b) is an explanatory diagram showing a capacitive propertyof a semiconductor switching element using a conventional siliconmaterial for comparison. In FIG. 3(a), the horizontal axis represents acollector-emitter voltage V_(CE) in the case of the SiCIGBT structure ora drain-source voltage V_(DS) in the case of the SiCMOS structure whilethe vertical axis represents a capacitive value. In FIG. 3(b), thehorizontal axis represents a collector-emitter voltage V_(CE) in thecase of an Si IGBT structure or a drain-source voltage V_(DS) in thecase of an Si MOSFET structure while the vertical axis represents acapacitive value. FIGS. 3(a) and (b) show an input capacitance Ciss, anoutput capacitance Coss, and a feedback capacitance Crss.

A thickness of the drift layer DFT of the SiC element (SiCIGBT, SiCMOS)shown in the cross-sectional views of FIGS. 2(a) and (b) is about 1/10of a thickness of the drift layer of the Si element (Si IGBT, Si MOSFET)with the same breakdown voltage. Therefore, in the SiC element, thefeedback capacitance Crss of the capacitive property, that is, acapacitive component proportional in an amount to the thickness of thedrift layer, becomes large. In general, in the SiC element, in order tosecure its electric properties such as the on-voltage and theon-resistance, a thickness of a gate insulating film Tox is alsodesigned to be thinner than that of the Si element. As a result, theinput capacitance Ciss proportional in an amount to the capacitance ofthe gate insulating film also increases. That is, when an SiC powermodule having the same current capacity with that of an Si power moduleis designed, the capacitive value of the SiC power module increases byseveral times as large as that of the Si power module as clearlyunderstood from comparison between FIGS. 3 (a) and 3 (b). For thisreason, if a gate drive circuit used in the Si power module is used as agate drive circuit for the SiC power module without carefulconsideration, there is a possibility of occurrence of malfunction suchas the decrease in the switching speed and the false turn-on phenomenon.

In order to prevent the malfunction, according to the presentembodiment, a circuit configuration of the gate drive circuit iscontrived, and besides, an operation sequence of the switching elementwhich is a constituent element is improved contrived. First, the circuitconfiguration of the gate drive circuit of the present embodiment willbe described with reference to FIG. 4 (FIGS. 5 to 7 are the same in thecircuit configuration).

The gate drive circuit according to the present embodiment is an exampleof the drive circuit that drives the SiCIGBT shown in FIGS. 1(a) and2(a). Obviously, it is needless to say that the example can be alsoapplied to the drive circuit that drives the SiCMOS shown in FIGS. 1(b)and 2(b). In FIG. 4, the SiCIGBT which is the semiconductor switchingelement SW is referred to as main switching element, and an SiCIGBThaving a reflux diode element SiCPND which is the semiconductor diodeelement D is described as an example.

As shown in FIG. 4, the gate drive circuit according to the presentembodiment includes a first switching element Q1, a second switchingelement Q2, a third switching element Q3, a fourth switching element Q4,and a capacitor C_(EXT).

A source of the first switching element Q1 is connected to a firstvoltage V_(PP), and a drain of the same is connected to the gateelectrode of the main switching element SW controlled by the gate drivecircuit. A source of the second switching element Q2 is connected to asecond voltage V_(EE), and a drain of the same is connected to the gateelectrode of the main switching element SW. A source of the thirdswitching element Q3 is connected to the first voltage V_(PP), and adrain of the same is connected to a first electrode of the capacitorC_(EXT). A source of the fourth switching element Q4 is connected to thesecond voltage V_(EE), and a drain of the same is connected to the firstelectrode of the capacitor C_(EXT) and to the drain of the thirdswitching element Q3. A second electrode of the capacitor C_(EXT) isconnected to the gate electrode of the main switching element SW.

Each of the first and third switching elements Q1 and Q3 is a PMOSFETand has a built-in diode element. Each of the second and fourthswitching elements Q2 and Q4 is an NMOSFET and has a built-in diodeelement. Each of the first and third switching elements Q1 and Q3functions as a pull-up circuit. Each of the second and fourth switchingelements Q2 and Q4 functions as a pull-down circuit. The third switchingelement Q3 and fourth switching element Q4 have respective functions ofassisting the charging/discharging in the input capacitance Ciss of themain switching element SW performed by the first switching element Q1and second switching element Q2. The capacitor C_(EXT) functions as astabilizing capacitor for preventing the false turn-on phenomenon.

The gate drive circuit according to the present embodiment includes acontrol circuit CTL. Each gate of the first, second, third, and fourthswitching elements Q1, Q2, Q3, and Q4 is connected to the controlcircuit CTL. The first, second, third, and fourth switching elements Q1,Q2, Q3, and Q4 controls the turning-on/off on the control circuit CTL.

The gate drive circuit according to the present embodiment includes agate-on resistor Rgon2, a gate-off resistor Rgoff2, a switching gate-onresistor Rgon1, a switch SWu, a switching gate-off resistor Rgoff1, anda switch SWd, as resistance elements that control the switching speed ofthe SiCIGBT that is the main switching element SW.

The gate-on resistor Rgon2 is connected between the drain of the firstswitching element Q1 and the gate electrode of the main switchingelement SW. The gate-off resistor Rgoff2 is connected between the drainof the second switching element Q2 and the gate electrode of the mainswitching element SW. The switching gate-on resistor Rgon1 and theswitch SWu which are connected in parallel with each other are connectedbetween the first voltage V_(PP) and the source of the first switchingelement Q1. The switching gate-off resistor Rgoff1 and the switch SWdwhich are connected in parallel with each other are connected betweenthe second voltage V_(EE) and the source of the second switching elementQ2. The switch SWu and switch SWd controls the turning-on/off on thecontrol circuit CTL.

The gate drive circuit according to the present embodiment hasresistance values which can be used independently by the turning-on/offoperations, respectively, and has a configuration in which these settingresistance values can be changed by the switches. By such aconfiguration, the switching speed can be easily controlled. As arelationship between the resistance values and the switching speed, whenthe resistance values are set to be small, the switching speed can belarge. Contrarily, when the resistance values are set to be large, theswitching speed can be small. By the large switching speed, the inputcapacitance Ciss of the main switching element SW can becharged/discharged in a short time.

For example, for the operation of turning the main switching element SWon, the gate-on resistor Rgon2 and the switching gate-on resistor Rgon1can be used. In this case, when the switch SWu connected in parallelwith the switching gate-on resistor Rgon1 is turned off, the resistancevalue is the resistance value obtained by the sum of the resistancevalue of the gate-on resistor Rgon2 and the resistance value of theswitching gate-on resistor Rgon1. When the switch SWu is turned on, theresistance value is only the resistance value of the gate-on resistorRgon2.

And, for the operation of turning the main switching element SW off, thegate-off resistor Rgoff2 and the switching gate-off resistor Rgoff1 canbe used. In this case, when the switch SWd connected in parallel withthe switching gate-off resistor Rgoff1 is turned off, the resistancevalue is the resistance value obtained by the sum of the resistancevalue of the gate-off resistor Rgoff2 and the resistance value of theswitching gate-off resistor Rgoff1. When the switch SWd is turned on,the resistance value is only the resistance value of the gate-offresistor Rgoff2.

In this manner, the present embodiment provides the gate-on resistorRgon2 and the switching gate-on resistor Rgon1 as the turn-on resistors,and provides the gate-off resistor Rgoff2 and the switching gate-offresistor Rgoff1 as the turn-off resistors. Thus, the present embodimentprovides a configuration which independently has a plurality of theturn-on resistors and a plurality of the turn-off resistors.

<Operation of Gate Drive Circuit>

Next, the operation of the gate drive circuit will be described withreference to FIGS. 4 to 7 and FIG. 8. FIGS. 4 to 7 are circuit diagramsshowing examples of operation modes (t1 to t4) of the gate drive circuitaccording to the embodiment, respectively. FIG. 8 is a waveform chartshowing an example of a timing chart of the gate drive circuit accordingto the embodiment. The operation mode (t1) shown in FIG. 4, theoperation mode (t2) shown in FIG. 5, the operation mode (t3) shown inFIG. 6, and the operation mode (t4) shown in FIG. 7 are performed so asto correspond to periods t1 to t4 of FIG. 8, respectively.

When the main switching element SW is in the off-state, the fourswitching elements as shown in FIG. 4, i.e., first, second, third, andfourth switching elements Q1, Q2, Q3, and Q4 are all in the off-state.

In order to turn the main switching element SW on, the first switchingelement Q1 is turned on first as shown in FIG. 8. By the turning-on ofthe first switching element Q1, the charging in the input capacitanceCiss (=“gate-collector capacitance Cgc” +“gate-emitter capacitance Cge”)of the main switching element SW is started so as to generate a chargecurrent Ig(Ciss). At this time, the fourth switching element Q4 is inthe off-state, and therefore, a charge current in the capacitor C_(EXT)which functions as the stabilizing capacitor for preventing the falseturn-on phenomenon is unnecessary. As a result, as shown in the periodt1 of FIG. 8, the total gate current Ig of the main switching element SWis suppressed to be small. If the capacitor C_(EXT) is electricallyconnected between the gate and the emitter of the main switching elementSW without the fourth switching element Q4, the total gate current Igincreases as shown by a broken line in FIG. 8.

The first switching element Q1 is turned on in the period t1, so thatthe charging in the input capacitance Ciss of the main switching elementSW advances, and the gate-emitter voltage Vge of the main switchingelement SW increases from −10 V through a terrace period (about 10 V) to15 V which is a desirable drive voltage. The voltage of −10 V is avoltage corresponding to the second voltage V_(EE), and 15 V is avoltage corresponding to the first voltage V_(PP).

As shown in FIG. 8, in the period t2 after the terrace period in theperiod t1, the third switching element Q3 is turned on. In this manner,as shown in FIG. 5, the third switching element Q3 assists a chargecurrent Ig(Cext) in the capacitor C_(EXT,) so that the input capacitanceCiss of the main switching element SW can be charged at a high speed.

Note that a timing of the turning-on of the third switching element Q3may be in the middle of the terrace period t1. Except for a region witha stable state of a supply current from the first voltage V_(PP), thatis, with the large total gate current Ig right after the turning-on ofthe first switching element Q1, even if the third switching element Q3is turned on, the assist current in the capacitor C_(EXT) can be stablysupplied.

In order to turn the main switching element SW off, as shown in theperiod t3 of FIG. 8, the third switching element Q3 is turned off, andthen, the first switching element Q1 is turned off while the secondswitching element Q2 is turned on, so that the discharging in the inputcapacitance Ciss of the main switching element SW is started. By suchcontrol, the capacitive value of the capacitor C_(EXT) can be obtainedto be apparently effectively small. As shown in FIG. 6, a dischargecurrent Ig(Ciss) serves as a drive current, and therefore, the totalgate current Ig of the main switching element SW at the turning-offstart is kept small as similar to the turning-on of the main switchingelement SW.

As shown in FIG. 8, in the period t4 after the terrace period in theperiod t5, the fourth switching element Q4 is turned on. In this manner,as shown in FIG. 7, the fourth switching element Q4 assists a dischargecurrent Ig(Cext) in the capacitor C_(EXT,) so that the input capacitanceCiss of the main switching element SW can be discharged at a high speed.

Note that a timing of the turning-on of the fourth switching element Q4may be in the middle of the terrace period t3. Except for a region witha stable state of a supply current from the second voltage V_(EE), thatis, with the large total gate current Ig right after the turning-on ofthe second switching element Q2, even if the fourth switching element Q4is turned on, the assist current in the capacitor C_(EXT) can be stablysupplied.

Since the fourth switching element Q4 is in the on-state after the mainswitching element SW is turned off, the capacitor C_(EXT) functions asthe stabilizing capacitor for preventing the false turn-on phenomenon.For this reason, even when main switching elements SW (SW1 u, SW2U, SW1v, SW2 v, SW1 w, SW2 w) of the opposed arms in the inverter circuit ofthe power conversion apparatus as described later (in FIG. 9) are turnedon and off at a high speed, the stable off-state can be maintained, andtherefore, the power conversion apparatus with high reliability can beprovided.

As described above, by using the circuit configuration of the gate drivecircuit and the operation sequence as described with reference to FIGS.1 to 8, the gate drive current can be suppressed. As a result, the falseturn-on phenomenon can be prevented.

<Power Conversion Apparatus>

FIG. 9 is an explanatory diagram showing an example of a configurationof the power conversion apparatus according to the embodiment. In apower conversion apparatus PT shown in FIG. 9, for example, theabove-described circuit configuration of the gate drive circuit andoperation sequence are applied to a so-called three-phase inverter.

As shown in FIG. 9, the power conversion apparatus PT according to thepresent embodiment includes the switching elements (main switchingelements) SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w, gate drivecircuits GD1 u, GD1 v, GD1 w, GD2 u, GD2 v, and GD2 w, a power-supplyvoltage VCC, a capacitor CO, and a load circuit LD.

In the power conversion apparatus PT of FIG. 9, each of the switchingelements SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w is a semiconductorswitching element composed of an n-channel type SiCMOS. As theseswitching elements SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w, notethat a semiconductor switching element using the SiCIGBT can be alsoused. A transistor switch unit can be formed of the switching elementsSW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w.

Each of reflux diode elements D1 u, D1 v, D1 w, D2 u, D2 v, and D2 w isconnected between the source and the drain of each of the switchingelements SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w. Each of thesereflux diode elements D1 u, D1 v, D1 w, D2 u, D2 v, and D2 w is formedof, for example, a Schottky barrier diode.

Each of the switching elements SW1 u, SW1 v, and SW1 w is arranged onthe upper arm side (high-voltage side P) while each of the switchingelements SW2 u, SW2 v, and SW2 w is arranged on the lower arm side(low-voltage side N). The switching elements SW1 u and SW2 u are for a Uphase, the switching elements SW1 v and SW2 v are for a V phase, and theswitching elements SW1 w and SW2 w are for a W phase. The switchingelements SW1 u, SW2 u, SW1 v, SW2 v, SW1 w, and SW2 w are paired on theupper arm side and the lower arm side, respectively, and three pairedswitching elements, each pair of which are for the U phase, V phase, orW phase, are provided.

Each of the switching elements SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, andSW2 w is provided with a sensing circuit that detects an overcurrent, anovervoltage, or a temperature of each of the switching elements althoughnot shown.

The gate drive circuits GD1 u, GD1 v, GD1 w, GD2 u, GD2 v, and GD2 w arethe gate drive circuits as shown in FIGS. 4 to 7, and drive theswitching elements SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, and SW2 w,respectively. As corresponded to the switching elements SW1 u, SW2 u,SW1 v, SW2 v, SW1 w, and SW2 w, the gate drive circuits GD1 u, GD2 u,GD1 v, GD2 v, GD1 w, and GD2 w are paired on the upper arm side and thelower arm side, respectively, and three paired switching elements, eachpair of which are for the U phase, V phase, or W phase, are provided.Note that these gate drive circuits may be collected into a singlecircuit.

Upon detecting an overcurrent flowing through each switching element, anovervoltage applied to each switching element, or overheating of eachswitching element, the sensing circuit outputs a sense signal SE (asshown in FIGS. 4 to 7). The sense signal SE which has been output fromthe sensing circuit is input to the control circuit CTL of the gatedrive circuit. Upon receiving the incoming sense signal SE, the controlcircuit CTL performs control for stopping the operations of all theswitching elements.

The power-supply voltage VCC and the capacitor CO are connected betweenone end (drain node) of the switching element on the upper arm side andone end (source node) of the switching element on the lower arm side.The respective gate drive circuits properly turns on and off thecorresponding switching elements, so that alternate-current signals withthe three phases (U phase, V phase, W phase) which are different fromone another are generated from the power-supply voltage VCC with adirect-current voltage. The load circuit LD is formed of, for example, amotor or others, and is properly controlled by the alternate-currentsignals with the three phases (U phase, V phase, W phase).

In this case, a detailed operation in the hard switching operation foreach of the U-phase, V-phase, and W-phase is the same as that shown inFIG. 8 and others. In the three-phase inverter, the switching element onthe upper arm side (such as the switching element SW1 u) shifts to theturn-on state as the switching element on the lower arm side (such asthe switching element SW2 u) is in the off-state.

At this time, the drain potential (VD) of the switching element on thelower arm side (such as the switching element SW2 u) increases up to alevel close to the power-supply voltage VCC. When the drain potential ofthe switching element on the lower arm side (such as the switchingelement SW2 u) rapidly increases, the gate potential of the switchingelement on the lower arm side (such as the switching element SW2 u)transiently increases.

However, in the power conversion apparatus according to the presentembodiment, the switching element Q4 is turned on by using theabove-described gate drive circuit to connect the capacitor C_(EXT)which functions as the stabilizing capacitor, and therefore, the falseturn-on phenomenon at the switching element (such as the switchingelement SW2 u) can be prevented. The gate drive current is alsoequalized, and therefore, scales of the gate drive circuit and the powerconversion apparatus using the gate drive circuit can be reduced, sothat the costs can be reduced.

The same goes for a case in which the switching element on the lower armside is the switching element SW2 v while the switching element on theupper arm side is the switching element SW1 v and a case in which theswitching element on the lower arm side is the switching element SW2 wwhile the switching element on the upper arm side is the switchingelement SW1 w.

<Alternate-Current Type Rail Vehicle>

FIG. 10 is an explanatory diagram showing an example of a configurationof an alternate-current type railway vehicle according to theembodiment. FIG. 10 exemplifies a circuit diagram showing an example ofa configuration of converter groups and a last-stage inverter that forma power converter in a configuration of a railway vehicle for analternate-current overhead wire.

As shown in FIG. 10, the alternate-current type railway vehicleaccording to the present embodiment includes a power converter 1, apantograph 2, and a motor (M3) 3. The power converter 1 includesconverter groups 10 (10-1 to 10-8) and a last-stage inverter 21.

FIG. 10 shows an example in which, for example, each of semiconductorswitching elements and semiconductor diode elements in eight-stageconverter groups 10 (FIG. 10 shows a converter group 10-1 out of theconverter groups 10-1 to 10-8) and a last-stage inerter 21 is made of anSiC compound material. FIG. 10 omits illustration of the gate drivecircuits that drive the semiconductor switching elements of theconverter groups 10 and last-stage inerter 21.

In the example of FIG. 10, the power converter 1 is used in a railwayvehicle to which is supplied with a plurality of alternate-currentpowers from a plurality of alternate-current overhead wires are input.This drawing shows examples of, for example, AC of 25 kV and AC of 15 kVin a single phase (1φ)) as the plurality of alternate-current powersfrom the plurality of alternate-current overhead wires. From theplurality of alternate-current overhead wires to the plurality ofconverters groups 10, the alternate-current powers are supplied throughthe pantograph 2 which is a current collector of the railway vehicle.

An output node of each of the plurality of converter groups 10 isshort-circuited, and the short-circuited node is connected to an inputnode of the last-stage inverter 21. The last-stage inverter 21 is theinverter that converts a direct-current power output from theshort-circuited node of each of the converter groups 10, into analternate-current power. To the output node of the last-stage inverter21, the motor 3 used for driving the railway vehicle is connected. Thismotor 3 is driven by an alternate-current power output from thelast-stage inverter 21. In this case, as the motor 3, for example, athree-phase motor driven by a three-phase (3φ)) alternate-current poweris exemplified.

Each converter group 10 configuring the power converter 1 includes afirst converter 11, a first inverter 12, a transformer 13, and a secondconverter 14. The first converter 11 converts an alternate-current powersupplied through the pantograph 2, into a direct-current power. Thefirst inverter 12 converts the direct-current power converted by thefirst converter 11, into an alternate-current power. The transformer 13converts the alternate-current power converted by the first inverter 12,into a predetermined alternate-current power. The second converter 14converts the predetermined alternate-current power converted by thetransformer 13, into a direct-current power. The direct-current powerconverted by the second converter 14 is supplied to the last-stageinverter 21, and the direct-current power is converted by the last-stageinverter 21 into an alternate-current power that drives the motor 3.

The first converter 11 is formed of switching elements SW11 to SW14 anddiode elements D11 to D14. Each of the switching elements SW11 to SW14is formed of the SiCMOS, and each of the diode elements D11 to D14 isformed of the SiC-SBD. As similar to the first inverter 12 and thesecond converter 14, each of switching elements SW21 to SW24 and SW31 toSW34 is formed of the SiCMOS, and each of diode elements D21 to D24 andD31 to D34 is formed of the SiC-SBD.

On the other hand, the last-stage inverter 21 which is connected to theshort-circuited output node of each converter group 10 is formed ofswitching elements SW51 to SW56 and diode elements D51 to D56, each ofthe switching elements SW51 to SW56 is formed of the SiCIGBT, and eachof the diode elements D51 to D56 is formed of the SiC-PND. It isdesirable to form each of the switching elements SW51 to SW56 and thediode elements D51 to D56 of the last-stage inverter 21 from a compoundmaterial. However, each of them can be formed from a silicon material.

In this manner, in the power conversion apparatus 1 shown in FIG. 10,each of the switching elements of the converter groups 10 is formed of aunipolar element having a wide band gap, and each of the switchingelements of the last-stage inverter 21 is formed of a bipolar elementhaving a wide band gap. As a power device material, attention is paid toa compound semiconductor such as SiC or GaN having a band gap largerthan that of silicon. Each of these compound semiconductor has a largeband gap, and therefore, has a breakdown voltage that is about 10 timesas high as a breakdown voltage of silicon. For this reason, a compounddevice is allowed to have a film thickness smaller than that of a Sidevice, and to have a significantly-low resistance value (Ron) at thetime of conduction. As a result, so-called conduction loss (Ron×i²)expressed by a product of the resistance value (Ron) and the conductioncurrent (i) can be reduced, and this manner can significantly contributeto improvement of the power efficiency. As attention is paid to suchadvantages as achieving the high breakdown voltage and the lowconduction loss, the switching elements and the diode elements using thecompound materials are used in the power converter 1 according to thepresent embodiment.

Contrivance is performed so as to use elements which are different in atype between the switching elements of the converter groups 10 and theswitching elements of the last-stage inverter 21. For example, by theusage of the unipolar elements as the switching elements of theconverter groups 10, an operation at a high frequency with a smallswitching loss can be achieved. The unipolar element has a high inputimpedance, and therefore, and has advantages which allows a weak voltageto be amplified with less noises and which has a high breakdown voltage.Meanwhile, by the usage of the bipolar elements for the switchingelements of the last-stage inverter 21, a large current can besupported.

When these switching elements are driven, the gate drive circuitaccording to the present embodiment has the small gate drive current,and therefore, can easily perform the high-frequency operation of theconverter groups 10. Further, the false turn-on phenomenon during thehigh-speed switching operation can be prevented, and therefore,reliability of electrical components which are under-floor components ofthe railway vehicle can be increased.

In the foregoing, the invention made by the inventors of the presentinvention has been concretely described based on the embodiments.However, it is needless to say that the present invention is not limitedto the foregoing embodiments, and various modifications and alterationscan be made within the scope of the present invention.

That is, it is needless to say that, if objects such as the reduction inthe gate drive current, the prevention of the false turn-on phenomenon,and the reduction in the power loss are achieved, various modificationscan be made within the scope of the present invention.

As each switching element, a compound device made of not only siliconcarbide (SiC) but also gallium nitride (GaN) may be applicable. When thecompound material is used as each switching element of the inverter orothers, it is needless to say that the power loss of the inverter can bereduced by usage of the switching element together with the gate drivecircuit.

It is needless to say that the same effects can be obtained in theapplication of the power conversion apparatus according to the presentembodiment to power systems for various types of usage. Typically,inverters of air conditioners, DC/DC converters of server powersupplies, power conditioners of photovoltaic power systems, inverters ofhybrid/electric cars, and others are cited.

Note that the present invention is not limited to the above-describedembodiments, and includes various modification examples. For example,the above-described embodiments have been explained for easilyunderstanding the present invention, but are not always limited to theone including all structures explained above.

Also, a part of the structure of one embodiment can be replaced with thestructure of another embodiment, and besides, the structure of anotherembodiment can be added to the structure of one embodiment. Further,another structure can be added to/eliminated from/replaced with a partof the structure of each embodiment.

EXPLANATION OF REFERENCE CHARACTERS

1 power converter

2 pantograph

3 motor

10 (10-1 to 10-8) converter group

11 first converter

12 first inverter

13 transformer

14 second converter

21 last-stage inverter

SW switching element

D diode element

Q1, Q2, Q3, Q4 switching element

C_(EXT) capacitor

V_(PP) voltage

V_(EE) voltage

CTL control circuit

SW1 u, SW1 v, SW1 w, SW2 u, SW2 v, SW2 w switching element

D1 u, D1 v, D1 w, D2 u, D2 v, D2 w diode element

GD1 u, GD1 v, GD1 w, GD2 u, GD2 v, GD2 w gate drive circuit

SW11 to SW14, SW21 to SW24, SW31 to SW34, SW51 to SW56 switching element

D11 to D14, D21 to D24, D31 to D34, D51 to D56 diode element

1. A gate drive circuit driving a main switching element, comprising: afirst switching element; a second switching element; a third switchingelement; a fourth switching element; and a capacitor, wherein a sourceof the first switching element is connected to a first voltage, and adrain of the first switching element is connected to a gate electrode ofthe main switching element, a source of the second switching element isconnected to a second voltage, and a drain of the second switchingelement is connected to the gate electrode of the main switchingelement, a source of the third switching element is connected to thefirst voltage, and a drain of the third switching element is connectedto a first electrode of the capacitor, a source of the fourth switchingelement is connected to the second voltage, and a drain of the fourthswitching element is connected to the first electrode of the capacitorand to the drain of the third switching element, and a second electrodeof the capacitor is connected to the gate electrode of the mainswitching element.
 2. The gate drive circuit according to claim 1,wherein the third switching element functions as a pull-up circuit, andthe fourth switching element functions as a pull-down circuit.
 3. Thegate drive circuit according to claim 1, wherein the third switchingelement is a PMOSFET, and has a built-in diode element, and the fourthswitching element is an NMOSFET, and has a built-in diode element. 4.The gate drive circuit according to claim 3, wherein the first switchingelement is a PMOSFET, and has a built-in diode element, and the secondswitching element is an NMOSFET, and has a built-in diode element. 5.The gate drive circuit according to claim 1 further comprising a turn-onresistor and a turn-off resistor which are independent of each other,wherein the turn-on resistor is connected between the drain of the firstswitching element and the gate electrode of the main switching element,and the turn-off resistor is connected between the drain of the secondswitching element and the gate electrode of the main switching element.6. The gate drive circuit according to claim 1 further comprising eachof a plurality of turn-on resistors and a plurality of turn-offresistors, wherein a first turn-on resistor of the turn-on resistors isconnected between the drain of the first switching element and the gateelectrode of the main switching element, a second turn-on resistor ofthe turn-on resistors is connected between the source of the firstswitching element and the first voltage, a first turn-off resistor ofthe turn-off resistors is connected between the drain of the secondswitching element and the gate electrode of the main switching element,and a second turn-off resistor of the turn-off resistors is connectedbetween the source of the second switching element and the secondvoltage.
 7. The gate drive circuit according to claim 6, wherein thesecond turn-on resistor is connected in parallel with a turn-on switch,a resistance value becomes a resistance value obtained by a sum of aresistance value of the first turn-on resistor and a resistance value ofthe second turn-on resistor when the turn-on switch is turned off, andthe resistance value becomes a resistance value of the first turn-onresistor when the turn-on switch is turned on, the second turn-offresistor is connected in parallel with a turn-off switch, and theresistance value becomes a resistance value obtained by a sum of aresistance value of the first turn-off resistor and a resistance valueof the second turn-off resistor when the turn-off switch is turned off,and the resistance value becomes a resistance value of the firstturn-off resistor when the turn-off switch is turned on.
 8. The gatedrive circuit according to claim 1 further comprising a control circuitcontrolling the first switching element, the second switching element,the third switching element, and the fourth switching element, wherein adriving timing of the third switching element and the fourth switchingelement controlled by the control circuit is after a terrace period ofthe main switching element.
 9. The gate drive circuit according to claim8, wherein, in order to turn the main switching element on, the firstswitching element is turned on, and then, the third switching element isturned on, while each of the second switching element and the fourthswitching element is in a turn-off state, and, in order to turn the mainswitching element off, the second switching element is turned on, andthen, the fourth switching element is turned on, while each of the firstswitching element and the third switching element is in a turn-offstate.
 10. The gate drive circuit according to claim 9, wherein chargingin an input capacitance of the main switching element is started byturning the first switching element on, and then, the charging in theinput capacitance of the main switching element is assisted by turningthe third switching element on, and discharging in the input capacitanceof the main switching element is started by turning the second switchingelement on, and then, the discharging in the input capacitance of themain switching element is assisted by turning the fourth switchingelement on.
 11. A power conversion apparatus comprising: three pairedfirst main switching elements and second main switching elements, eachpair of which are for a U phase, a V phase, or a W phase, and each pairof which are connected in series between a power supply on ahigh-voltage side and a power supply on a low-voltage side; and threepaired first gate drive circuits and second gate drive circuits, eachpair of which are for the U phase, the V phase, or the W phase, and eachpair of which alternately turn each of the three paired first and secondmain switching elements on and off, wherein each of the first gate drivecircuits and the second gate drive circuits includes: a first switchingelement; a second switching element; a third switching element; a fourthswitching element; and a capacitor, a source of the first switchingelement is connected to a first voltage, and a drain of the firstswitching element is connected to a gate electrode of the first mainswitching element or to a gate electrode of the second main switchingelement, a source of the second switching element is connected to asecond voltage, and a drain of the second switching element is connectedto the gate electrode of the first main switching element or to the gateelectrode of the second main switching element, a source of the thirdswitching element is connected to the first voltage, and a drain of thethird switching element is connected to a first electrode of thecapacitor, a source of the fourth switching element is connected to thesecond voltage, and a drain of the fourth switching element is connectedto the first electrode of the capacitor and to the drain of the thirdswitching element, and a second electrode of the capacitor is connectedto the gate electrode of the first main switching element or the gateelectrode of the second main switching element.
 12. A railway vehiclehaving a power converter driving a three-phase motor, wherein the powerconverter includes an inverter converting a direct-current power, whichhas been created by converting an alternate-current power input from analternate-current overhead wire, into an alternate-current power to besupplied to the three-phase motor, the invertor includes: three pairedfirst main switching elements and second main switching elements, eachpair of which are for a U phase, a V phase, or a W phase, and each pairof which are connected in series between a power supply on ahigh-voltage side and a power supply on a low-voltage side; and threepaired first gate drive circuits and second gate drive circuits, eachpair of which are for the U phase, the V phase, or the W phase, and eachpair of which alternately turn the three paired first and second mainswitching elements on and off, and each of the first gate drive circuitsand the second gate drive circuits includes: a first switching element;a second switching element; a third switching element; a fourthswitching element; and a capacitor, a source of the first switchingelement is connected to a first voltage, and a drain of the firstswitching element is connected to a gate electrode of the first mainswitching element or to a gate electrode of the second main switchingelement, a source of the second switching element is connected to asecond voltage, and a drain of the second switching element is connectedto the gate electrode the first main switching element or to the gateelectrode of the second main switching element, a source of the thirdswitching element is connected to the first voltage, and a drain of thethird switching element is connected to a first electrode of thecapacitor, a source of the fourth switching element is connected to thesecond voltage, and a drain of the fourth switching element is connectedto the first electrode of the capacitor and to the drain of the thirdswitching element, and a second electrode of the capacitor is connectedto the gate electrode of the first main switching element or to the gateelectrode of the second main switching element.
 13. The railway vehicleaccording to claim 12, wherein the power converter includes a pluralityof converter groups each including: a first converter converting thealternate-current power, which has been input from the alternate-currentoverhead wire, into the direct-current power; a first inverterconverting the direct-current power, which has been converted by thefirst converter, into an alternate-current power; a transformerconverting the alternate-current power, which has been converted by thefirst inverter, into a predetermined alternate-current power; and asecond converter converting the alternate-current power, which has beenconverted by the transformer, into a direct-current power, and eachoutput of the plurality of converter groups is short-circuited, and isconnected to an input of the inverter converting the direct-currentpower into the alternate-current power to be supplied to the three-phasemotor.